It is generally considered that spatial complementarity would determine the specificity of antigen-antibody interactions. On the other hand, it is also reported that substitution of amino acids in a distant region from the binding site profoundly affects the antigen-antibody interaction. Such mutual influence between distant regions is very characteristics of the hypothetical global cooperative interactions maintaining the three-dimensional structure of proteins (see the other reports from this Section). Thus the two phenomena might be closely related. Being consistent with this view, studies using lH-2H exchange, show that a polyclonal antibody exhibits a reduced motility and an increased stability without a large change in conformation upon binding with a hapten. Thus, we propose to investigate the specificity of antigen-antibody interaction in relation to the global cooperative interactions as follows. l) A three-fragment complex of yeast cytochrome c will be developed. 2) A monoclonal antibody will be prepared to yeast cytochrome c. Then, the antigenic site will be determined using the fragment complex. 3) Thermodynamic and kinetic parameters of the antibody-antigen reaction with the fragment complex will be determined as a function of substitution of amino acids of the antigen using chemical synthesis. 4) Change in molecular motion of antibody upon binding with antigen containing unsubstituted and substituted amino acids will be determined using lH-3H or H-2H exchange. The degree of these changes will be related to the strength of binding with antigen. 5) A fragment exchange technique will also be used to investigate global influence on the antibody of antigen-antibody interaction. 6) Gene manipulation will be contemplated to investigate the effect of substitution of amino acids of variable domain of light chains (and heavy chain, if possible) on the antigen-antibody interaction.